Eukaryotic cells must temporally coordinate diverse cellular processes to ensure the fidelity of cell proliferation. Defects in the execution or temporal coordination of cell cycle processes increase genetic instability and promote cancer. Thus, it is important to resolve the mechanisms of conserved cell cycle regulations. The Saccharomyces cerevisiae Mitotic Exit Network (MEN) is essential for coordinating several cell cycle processes during mitotic exit, including cyclin dependent kinase (CDK) inactivation, cytokinesis, mitotic spindle disassembly and activation of G1 gene expression. The Mob1-Dbf2 kinase complex is a key component of MEN and is orthologous to the human Mob1A-LATS kinase complex, which functions as mammalian tumor suppressor. Yeast Mob1-Dbf2 has been implicated in cytokinesis, activation of Cdc14 phosphatase (which enables mitotic exit by inactivating CDK phosphorylations) and mitotic spindle function via regulation of Aurora kinase. Nevertheless, the functional mechanisms and substrates of yeast and mammalian Mob1-Dbf2 kinases are not known. The goal of the proposed work is to determine how the Mob1-Dbf2 kinase complex temporally coordinates cytokinesis with mitotic exit and chromosome segregation. We propose three aims. The first aim is to determine the role of Mob1-Dbf2 in cytokinesis. We will conduct cell biological, biochemical and genetic approaches to test the hypothesis that MEN controls cytokinesis by regulating targeted membrane deposition and by activating the RAM (Regulation of Ace2 transcription factor and polarized Morphogenesis) signaling network, which controls cytokinesis, gene expression and other functions. The second aim is to determine the role of the Mob1-Dbf2 kinase complex in Cdc14 phosphatase activation during mitotic exit and cytokinesis. We will test the hypothesis that Mob1- Dbf2 kinase directly induces Cdc14 phosphatase release from the nucleolus and will define the epistatic relationship of Mob1-Dbf2 and Cdclp with respect to cytokinesis. The third aim is to determine the role of Mob1-Dbf2 in mitotic checkpoint signaling. We will test the hypothesis that Mob1-Dbf2 is required for Aurora-dependent checkpoint signaling. Given the conservation of Mob1-Dbf2, the proposed work will help resolve the function of all MEN-related signaling networks, including the human hMob1A-LATS1 tumor suppressor pathway, and will help elucidate the underlying mechanisms of cancer development.